Widow tint numbers refer to the % of light transmission, so they are saying you can do a 70% tint which isn't much at all. Plus, they seem to be focusing on this section of the vehicle code:

(2) The window glazing with the material applied meets all requirements of Federal Motor Vehicle Safety Standard No. 205 (49 C.F.R. 571.205), including the specified minimum light transmittance of 70 percent and the abrasion resistance of AS-14 glazing, as specified in that federal standard.

But ignoring the section directly before:

(1) The material has a minimum visible light transmittance of 88 percent.

From my understanding the window tint film itself would have to be at least an 88% tint and the film plus the glass no more than %70.

I think this is why most shops just say none.

William.